Not applicable.
Not applicable.
1. Field of the Invention
The present invention relates, in general, to radar imaging methods, and in particular, to apparatus and methods for operating satellites utilizing radar and holography for imaging an orbited planet. The method of the present invention is also applicable for use in non-satellite imaging applications such as medical imaging.
2. Information Disclosure Statement
It is often desired to image a planet""s surface and subsurface with high resolution in near real time. Well-known solutions for this problem include Synthetic Aperture Radar (xe2x80x9cSARxe2x80x9d) using microwave imaging. Known satellite SAR focus a xe2x80x9cflat earthxe2x80x9d field of view (xe2x80x9cFOVxe2x80x9d) to a flat physical receiving antenna. Additionally, they cycle their complex imaging received signal at the rate in which the Doppler history (phase) on the physical aperture (the antenna) xe2x80x9cfillsxe2x80x9d, i.e., when the finest phase replica is first present on the face of the antenna. At this time, known SAR convert from analog to digital (A/D) while simultaneously detecting phase, and then digitally focus and truncate series expressions of the xe2x80x9cflat earthxe2x80x9d geometry, in order to linearize and orthogonalize the imagery set, thereby inserting accumulating bias errors.
Unlike Doppler, which is coherent, Range is not coherent. Additionally, because prior art SAR technology uses Range for the second dimension, which is the xe2x80x9cweak linkxe2x80x9d in its technology (limiting the finest resolution and causing the largest burden of noise), prior art SAR necessarily uses extremely wide bandwidth and is the principal cause of excessive RF power requirements at the satellite. RF propagation losses and realistic antenna beam widths force the wide bandwidth prior art SAR satellite to be limited to use at low altitudes causing associated infrequent revisit intervals, and xe2x80x9cstore and forwardxe2x80x9d imaging data is thereby forced to be downlinked at infrequent intervals, overloading the downlink capacity and limiting the overall effectiveness of prior art SAR technology. Prior art SAR technology also has very small swath widths that necessarily limit the number of available imaged areas, making prior art SAR technology unacceptable for use as a commercial service.
Known prior art interferometric imaging technology necessarily focuses outward into space because the mensurational accuracy required is too demanding for downward looking, earth oriented, fine resolution imaging using known prior art technology.
It is therefore desirable to have an improved satellite imaging system that does not have these problems found in the prior art. It would be desirable to have an improved satellite imaging system that has substantially improved gain and signal-to-noise ratio as compared to the prior art, and that further has a wide FOV swath and whose image reconstruction is decoupled from a dependence on time. It is further desirable to have an improved satellite imaging system with substantially better phase closure accuracy than heretofore possible.
Grisham, U.S. Pat. No. 3,243,706 (issued Mar. 29, 1966; hereinafter, the xe2x80x9cROSAE patentxe2x80x9d), describes a satellite system having three subsystems of two pair of satellites each, and the orbits of all satellites within each subsystem are nominally circular. In one subsystem, the two pair of satellites orbit circularly in an equatorial plane. The other two subsystems have polar planes of circular orbit, with the polar planes being orthogonal to each other and also being orthogonal to the equatorial plane subsystem so that the planes of all three subsystems are mutually perpendicular. Within each subsystem, the two members of one pair of satellites are nominally 180xc2x0 apart and orbit in one sense (direction), while the two members of the other pair of satellites are nominally 180xc2x0 apart but orbit in the other sense (direction). While the satellite configuration of the ROSAE patent is a preferable configuration for use by the present invention, the ROSAE patent does not disclose or suggest using the microwave interferometry radiating incrementally accumulating holography (xe2x80x9cMIRIAHxe2x80x9d) method of the present invention in combination with the satellite configuration of the ROSAE patent.
Caputi, U.S. Pat. No. 4,325,065 (issued Apr. 13, 1982), describes a process for correcting data from a bistatic synthetic aperture radar (xe2x80x9cSARxe2x80x9d) to eliminate distortions and resolution limitations due to the relative positions and motions of the radar transmitter and receiver with respect to a target.
Grisham, U.S. Pat. No. 4,602,257 (issued Jul. 22, 1986; hereinafter the xe2x80x9cSARAH patentxe2x80x9d) and fully incorporated herein by reference, describes a method of satellite operation utilizing a paired-satellite configuration in which one satellite illuminates the imaged field of view and the other satellite receives the reflected energy using bistatic synthetic aperture radar (xe2x80x9cSARxe2x80x9d), but did not teach or suggest the use of interferometers for illumination or holography for recording the image data, and thus did not generate a large positive Gain spatial matched filter in a Fourier plane (i.e., a hologram). Instead, the SARAH patent taught use of Range/Doppler for illumination, and generated a time dependent matched filter in the Fourier plane. Because both SAR and SARAH use time referencing, image reconstruction by these prior art methods is necessarily dependent on time.
None of the known prior art references, either singly or in combination, disclose or suggest the present invention.
The present invention is a satellite architecture used to create a narrow-bandwidth actively-illuminated interferometric Synthetic Aperture Radar (xe2x80x9cSARxe2x80x9d), specifically, a bistatic SAR, whose Very Long Baseline Interferometer (xe2x80x9cVLBIxe2x80x9d) has a baseline between its two bistatic apertures, each on a different satellite, that is considerably longer than the diameter of the field of view (xe2x80x9cFOVxe2x80x9d). This is in contrast to prior art bistatic SAR where the interferometer baseline is shorter than the diameter of the FOV because both bistatic apertures were on the same satellite. The preferred embodiments of the invention use subsets of the satellite orbit configuration as described in Grisham, U.S. Pat. No. 3,243,706 (issued Mar. 29, 1966; hereinafter, the xe2x80x9cROSAE patentxe2x80x9d) and fully incorporated herein by reference, whose satellite orbit structure is shown in FIG. 1.
Each of the preferred embodiments of the present invention has one or more VLBI created by pairs of satellites. The most preferred embodiments, having symmetrical configurations of three, six, and twelve satellites, are built on a foundation of Very Large Array (xe2x80x9cVLAxe2x80x9d) satellite VLBI triads, with each satellite of the triad being in its own nominally circular orbit, with the orbital planes of the three satellites of the triad being mutually orthogonal, and with the orbital angular velocity of each satellite preferably being five times the angular rotational velocity of the earth. For each VLA triad of satellites, VLBI pairs are formed by pairwise grouping of the satellites in the VLA, with the third satellite of the VLA being used as a control satellite to receive Michelson interferometric data from the VLBI pair to maintain phase closure, and also to receive Fizeau interferometric imaging data from the VLBI pair to be recorded in the Fourier plane of a holographic disc.
In contrast to prior art SAR technology, in which the synthetic aperture is time referenced, the present invention extends the synthetic aperture in size and in a second dimension and uses active illumination of the FOV by interferometers, thereby causing the resulting synthetic aperture to become spatially referenced. In terms of matched filter theory, the present invention""s microwave interferometry radiating incrementally accumulating holography (xe2x80x9cMIRIAHxe2x80x9d) technology provides a two-dimensional spatial matched filter with an extraordinarily narrow passband (for finer resolution and higher gain than heretofore possible). Like all interferometers, the interferometers of the present invention are self-referencing, i.e., referenced as a function of their spatial position, with resolution improving as the length (and frequency) of the interferometer increases. Accordingly, the coherent gain produced by the method and apparatus of the present invention not only increases with the size (VLBI length) of its synthetic aperture, but the gain coefficient of the present invention is also squared, as compared with the gain coefficient of prior art SAR technology, because the present invention xe2x80x9cfully fillsxe2x80x9d a two-dimensional synthetic aperture by xe2x80x9cfully fillingxe2x80x9d a two-dimensional spatial matched filter (hologram).
A xe2x80x9cMasterxe2x80x9d Stable Oscillator (xe2x80x9cSTALOxe2x80x9d) or clock is preferably used to slave the STALO of every other satellite throughout the architecture of the present invention. In this way, in the preferred embodiments, each set of three VLBI, arranged as an equilateral triad, act in concert as a single coherent VLA. A preferred three-satellite configuration (xe2x80x9cMIRIAH*3xe2x80x9d) has one such VLA triad of VLBI. A preferred six-satellite configuration (xe2x80x9cMIRIAH*6xe2x80x9d) has eight such VLA triads of VLBI, each rotating in one sense. A preferred twelve-satellite configuration (xe2x80x9cMIRIAH* 12xe2x80x9d), using the satellite orbital configuration of the ROSAE patent, has sixteen such VLA triads of VLBI with eight VLA triads rotating in one sense and with the remaining eight VLA triads rotating in the other sense.
For the example parametric values used in the conceptual parametric analysis presented hereinafter in the detailed description of the preferred embodiments, the minimum bandwidth BCohInt, corresponding to the total coherence time of one Sidereal Day (86,164 seconds) of the spatial matched filter (hologram) is shown to be about 3.646xc3x9710xe2x88x925 Hz, which rejects the bistatic Doppler shift signal while passing the interferogram data. The minimum deviation between the interferogram diffraction lines at the at the edge of each zone plate in the Fourier plane is shown to shrink with coherence time, thereby creating Zone Plates (Fresnel lens in the Fourier plane) that are concentric about each pixel in the image plane. Accordingly, the matched filter is actually a hologram comprised of a collage of millions of these Fresnel lenses, wherein the deviation distance between the outer fringes of these Fresnel lenses sets the resolution, and the Gain is set by the ratio of the Fresnel lens area to the pixel area. Even though the VLA is a synthetic aperture, this resulting gain is shown to be the same as that derived by computing the Gain of the VLA as a real aperture.
The conceptual parametric analysis shows that the coherent Gain for the VLA triads is computed in the usual way, as for real apertures, with the xe2x80x9csweptxe2x80x9d area, AVLA, of the VLA (i.e., the synthetic aperture area xe2x80x9cfilledxe2x80x9d during the coherence time Tcoh) being given by             A      VLA        ⁡          (      t      )        =      π    ·                  (                              c            ⁡                          (              t              )                                            2            ⁢                          xe2x80x83                        ⁢                          cos              ⁡                              (                                  π                  6                                )                                                    )            2      
where c(t), shown in FIG. 39, is the magnitude (length) of the VLBI vector C(t) as shown in FIG. 38, such that the two-dimensional Synthetic Aperture Gain, GSynAp(t), of the VLA is then given by the well-known formula             G      SynAp        ⁢          (      t      )        =            4      ⁢              xe2x80x83            ⁢              π        ·                              A            VLA                    ⁢                      (            t            )                          ·        η                    λ      2      
and as shown graphically in FIG. 28, where xcex is the wavelength and xcex7 is the antenna efficiency. For the typical test case values used for purposes of evaluating the conceptual parametric analysis, it should be noted that this gain for the present invention is enormous, being on the order of 1016, which will be understood to be an extraordinary improvement over the prior art.
As shown in FIGS. 3 and 4, for each VLBI created by a satellite pair A-B of a VLA triad, there is a xe2x80x9ccontrolxe2x80x9d satellite C of the VLA triad, not to be confused with the xe2x80x9cMasterxe2x80x9d satellite that provides the master STALO clock to which all satellites in the architecture are slaved, with each xe2x80x9ccontrolxe2x80x9d satellite being substantially along the VLBI phase centerline and equidistant from the other two satellites of the VLA. It shall be understood that each VLA triad has three VLBI, one for each satellite pair of the VLA triad, so that the role of xe2x80x9ccontrolxe2x80x9d satellite is occupied by each satellite of the VLA, respectively for the VLBI on whose phase centerline the satellite sits. Using the MIRIAH*6 configuration shown in FIG. 2 as an example, there will be the need for 24 xe2x80x9ccontrolxe2x80x9d satellites for the eight VLA triads shown, but only one xe2x80x9cMasterxe2x80x9d satellite. Note that, for the MIRIAH*6 configuration, each satellite serves as a xe2x80x9ccontrolxe2x80x9d satellite for each of the four VLA triads of which it is a member, such that the six satellites of the MIRIAH*6 architecture, each serving the xe2x80x9ccontrolxe2x80x9d satellite role for each of the four VLA triads of which it is a member, together serve the required 24 xe2x80x9ccontrolxe2x80x9d satellite roles. It will be understood that this architecture extends naturally to the MIRIAH*12 and MIRIAH*3 configurations, in a manner that will now be apparent to those skilled in the art.
The functional block diagram systems architecture of each VLBI is shown in FIG. 18. As described more fully in the detailed description of the preferred embodiments, in contrast to prior art SAR technology, phase coherence of the imaging data is preserved by the present invention up to the Fourier plane. Additionally, and also in contrast to the prior art, the second Power-Aperture, namely; the optical laser xe2x80x9creadxe2x80x9d of the previously-written hologram, need not be either phase locked to the STALO, or even of the same frequency used to xe2x80x9cwritexe2x80x9d the hologram, because phase information is preserved as a diffraction pattern in the Fourier plane of the hologram during the extended xe2x80x9cwritexe2x80x9d of the hologram. If a different frequency is used in illuminating the hologram during the xe2x80x9creadxe2x80x9d transformation to the image plane, it will be understood that the focal point will shift, the scale will change, and the SNR will then become a squared function of the two frequency ratios. However, the extraordinary Gain, SNR, and energy density improvements of the present invention will be maintained.
The present invention is a satellite architecture that is primarily designed for near real-time, day or night, all weather, fine resolution imaging of the earth""s surface and subsurface. Secondarily, this invention will enable earth crust stress imaging, prediction of earthquakes and volcano episodes, and imaging of moving objects such as aircraft, trucks, etc. In addition to applications such as imaging of the earth, the methods of the present invention can also be used for penetration imaging of human and animal bodies for medical applications. The invention is designed to have the ideal attributes needed to provide a global, profitable, commercial imaging service that can provide fresh updates, day or night, in all weather, penetrating foliage and other obscurants including permeable soils. The microwave imagery will have high contrast and fine resolution, will be accessible globally on an open-demand basis, and will be hyperspectral (in numerous separate channels) and diverse in polarity. The architecture of the present invention permits much larger Fields of View (xe2x80x9cFOVxe2x80x9d) than heretofore possible thereby ensuring an adequate supply of imagery data.